A blood sample tested in Seoul found high levels of a poison to kill a person

As the snow falls and the wind bites on a stark winter's day in Seoul, a lady rugged up in a green woolen overcoat and scarf arrives at a church nestled among shops and office buildings.

Kim Ha-young comes in from the cold and greets me with a warm and gentle smile. She shakes my hand - her gold wedding band still on display. It's not her real name but she can't afford to reveal her true identity: She believes that would place her family in serious danger and she's lost far too much already.

It's almost two years since she found her husband, Kim Chang-hwan, foaming at the mouth in the Chinese city of Dandong on the North Korean border. The 46-year-old father of two had been working as a missionary, helping North Korean defectors escape across the border. Kim Ha-young was living in the border city as well, helping her husband. She had just spoken to her husband 15 minutes earlier.

"He told me he was meeting a North Korean defector and would then come home. A short time later I got a call from one of his colleagues who said (my husband) collapsed on the street and he told me to rush to the hospital," she said. "When I got there he was dead."

Hospital officials said Kim Chang-hwan had committed suicide by swallowing pesticides. His wife believes he was killed by a North Korean agent.

"My husband was aware of the risk. People around us were telling us that it is a dangerous job because North Korean government will severely deal with people who helped North Koreans defect," she said.

"But I did not think that my husband would never come back. We never expected that it would cost a life. We thought maybe the risk was prison or being expelled from the country by Chinese government. We never thought that it would cost his life."

Refusing to accept the hospital's explanation for her husband's death, Kim Ha-young demanded the Chinese government conduct an autopsy. The autopsy report came back saying there was no poison in his system. Fearful of a cover-up, she went to the morgue before his body was cremated and collected samples of his blood on a glove and gave them to South Korean authorities on her return to Seoul.

The government report on that blood sample, reviewed by CNN, revealed levels of poison high enough to kill a person instantly.

Fellow missionary Seok Sa-hyun said his friend had received threats in the past but nothing could stop him from helping defectors or providing food to North Korea's malnourished children.

"Mr. Kim established a food company and factory in Dandong and he delivered tofu, noodles, bread and baby milk to North Korea. Mr. Kim provided food and spread God's word, and so North Korea was threatened by it and put him at the top of their 'terrorist list' -- I'm certain they poisoned him."

One man was arrested on espionage charges in connection to the murder and sentenced to four years in prison, but it's little comfort to his grieving wife.

"He was too young to die. We had so many plans and dreams. He was taken away from us ... in such a cruel way."